1. Field of the Invention
The present invention is related to the area of optical instruments and, in particular, related to an optical performance monitor for precise wavelength, power and signal-to-noise ratio measurement.
2. Description of Related Art
The future communication networks demand ever increasing bandwidths and flexibility to different communication protocols. DWDM (Dense Wavelength Division Multiplexing) is one of the key technologies for such optical fiber communication networks. DWDM employs multiple wavelengths or channels in a single fiber to transmit in parallel different communication protocols and bit rates. Transmitting several channels in a single optical fiber at different wavelengths can multi-fold expand the transmission capacity of the existing optical transmission systems, and facilitating many functions in optical networking.
Optical telecommunications generally involves the use of light beams propagating through optical networks to transmit data from one end to another end. If there are any deficiency in the optical networks (e.g., fibers or switches), the quality or parameters of the light beams will be degraded. Thus monitoring the performance of optical networks is important in optical communications.
The performance of a DWDM optical communication network can be monitored by measuring the wavelengths, powers and signal-to-noise ratios of its channels in real-time. Diffraction grating and tunable filters are most commonly used techniques for DWDM network performance monitoring. Wavelength references are usually used to remove wavelength uncertainties associated with tunable filters. Fabry-Perot tunable filters with different wavelength reference techniques have been used in the past (e.g., see U.S. Pat. No.: 5,838,437). With tunable filters, it is potentially to obtain finer wavelength resolutions and to have tapped signals for Bit-Error-Rate Test (BERT).
Because of uncertainties of mechanical scanning, real-time wavelength calibrations are commonly used to ensure the wavelength accuracy. Usually, optical-mechanical switches are used to switch back and forth between the optical signal channels under monitoring and the optical wavelength reference channel for referencing. Wavelength reference techniques can be a series of Fiber Bragg Grating (FBG) filters or a slope filter. Gas cell wavelength references have also been used for DWDM applications. In those applications, gas cells covering the same wavelength as the communication signal channel wavelengths are often used (e.g., see Duwayne Anderson, et al, xe2x80x9cReal-time wavelength calibration with picometer accuracy in swept-laser system,xe2x80x9d Technical Proceedings, NFOEC 2001, Vol. 2, 1089-1100, 2001).
New techniques for simple and accurate determination of wavelength(s) in optical signals are still desirable in the field.
The present invention pertains to method and apparatus for wavelength control and measurement. In one aspect of the present invention, an optical signal to be measured is tapped off a portion referring to as a source signal. Together with a reference signal, the source signal is coupled to a tunable filter. The passing bands of the tunable filter is so controlled that one wavelength xcexx from the source signal and one wavelength xcexrx from the reference signal transmit through. Relying on a band separation filter, the wavelength xcexrx is separated from the source signal and then coupled to a gas cell of a known spectrum, a filtered signal of the wavelength xcexrx is then coupled to a photo-detector for subsequent electrical measurement.
Because the intensities of the source signal and the reference wavelength absorbed by the gas in the gas cell are sampled simultaneously, the wavelength xcexx of the source signal can be calculated accordingly from mapping the sampled gas absorption spectrum at the same time interval. The relationship is a one-to-one mapping between two different Free Space Ranges (FSR) and can be uniquely defined by the properties of the tunable filter. As a result, wavelength, power, and signal-to-noise ratio in an optical system (e.g., a DWDM system) can be precisely determined.
One of the objects in the present invention is to provide a new technique for wavelength control and measurement with a tunable filter and a gas cell filter.